1. Field of the Invention
The present invention relates to a power converter using a plurality of parallel circuits composed of switching devices and diodes and a motor driving device using the power converter, and more particularly to an inverter circuit in which a Schottky barrier diode (SBD) of large bandgap semiconductors such as SiC, and GaN, etc. is used for a diode connected to a power semiconductor switching device in parallel.
2. Description of the Related Art
In recent years, semiconductor devices made of a silicon carbide (SiC) or a gallium nitride (GaN) have attracted attention as wide gap semiconductor devices. Since these materials have dielectric breakdown field strengths which are about ten times larger than that of a silicon (Si) and the thickness of a drift layer to keep resistance to voltage can be reduced to about one-tenth of that of a silicon (Si), an on-voltage of a power device can be lowered. For this reason, even in a high-voltage domain in which only a bipolar device can be used if the device is made of Si, a unipolar device can be used if the device is the wide gap semiconductor device made of SiC, etc.
In a power semiconductor module used for an inverter circuit, a free wheel diode is connected to a switching device in parallel. In a conventional power semiconductor module, a Si-PiN diode is used as the free wheel diode. The Si-PiN diode is a bipolar semiconductor device, and is configured so that a conductivity modulation causes a voltage drop to be small when a large current is supplied in a forward bias direction. However, the Si-PiN diode has the property of generating carriers remained in the Si-PiN diode as a reverse recovery current by the conductivity modulation during a process from a forward bias condition to a reverse bias condition. In the Si-PiN diode, since a lifetime of the remaining carrier is long, the reverse recovery current becomes large. For this reason, there arises a problem that this reverse recovery current causes a turn-on loss (Eon) and a recovery loss (Err) at the time of reverse recovery of a diode to be large.
On the other hand, a Schottky barrier diode (SBD) is an unipolar semiconductor device. Since few carriers are generated by the conductivity modulation and the reverse recovery current is very small, the turn-on loss and the recovery loss can be reduced when the SBD is used in an inverter circuit. Also, the conventional Si has a low dielectric breakdown field strength, and a large resistance is generated when the SBD is energized if the SBD is made to have a high voltage structure. Therefore, a voltage limit of a Si-SBD is about 200 V. However, since the SiC has the dielectric breakdown field strength which is ten times larger than that of the Si, a high-voltage SBD can be realized. Therefore, it is well known that the turn-on loss (Eon) and the recovery loss (Err) at the time of reverse recovery of a diode can be reduced.
Also, in a main circuit of an inverter of a power semiconductor module using a conventional Si-PiN diode, a commutation surge voltage (ΔVp=L*reverse recovery di/dt) obtained by product of change in reverse recovery current of the Si-PiN diode at the time of attenuation (reverse recovery di/dt) and an inductance L of the main circuit is added. If the sum (E+ΔVp) of a power supply voltage (E) and a surge voltage (ΔVp) is greater than a withstand voltage of the power semiconductor switching device, the power semiconductor device may be destroyed. For this reason, various techniques for reducing the inductance and noise of the main circuit have been proposed.
For example, JP 2009-225570 A discloses a technique for reducing the noise by connecting an impedance between a terminal of a free wheel diode connected to a switching device in parallel and a terminal of the switching device.
JP 2008-092663 A discloses a technique for reducing the noise by changing a gate resistance in a driving circuit of IGBT (Insulated Gate Bipolar Transistor) during a turn-on period.
By the way, as described above, since the reverse recovery current does not flow through the Schottky barrier diode made of SiC (SiC-SBD), etc., the turn-on loss and the recovery loss are essentially small. Also, since there is no reverse recovery di/dt, it is possible to increase a switching speed (turn-on speed) of the switching device by decreasing the gate resistance of the power semiconductor switching device. For this reason, it is possible to reduce the loss further.
However, the SiC-SBD has some junction capacitance. Therefore, when the switching device is turned on, the power supply voltage is applied to both terminals of the diode, and a resonant current flows caused by the junction capacitance of the diode and the inductance of the main circuit. That is, the SiC-SBD has a disadvantage that a large surge voltage (ringing voltage) is applied to both terminals of the diode, so that a ringing noise is increased.
Also, in an EMC (Electro-Magnetic Compatibility) noise measurement for an electromagnetic interference (EMI) countermeasure, a target frequency is changed, and a spectral distribution for indicating a noise intensity of the target frequency is measured. This spectral distribution can be calculated by using FFT (Fast Fourier Transform) in at least low frequency domain. In the calculation, the noise intensity (power) is sequentially integrated for a predetermined period.
For this reason, in order to reduce the noise generated by an apparatus, a peak value of the noise intensity is not necessarily suppressed, an average power within a predetermined period may be decreased.